A polymer film (hereinafter referred to as film) has advantages such as excellent light transmission properties and flexibility. Accordingly, the film is widely used as an optical functional film. As a representative of the film, a cellulose ester film using cellulose acylate or the like has excellent toughness, and phase difference is small in the cellulose ester film. Therefore, the cellulose ester film is utilized as a base of photosensitive material. Additionally, the cellulose ester film is utilized as a protective film in a polarizing filter and an optical compensation film as components of a liquid crystal display (LCD) whose market is increasingly expanded recently.
As a film production method, mainly, there are a melt-extrusion method and a solution casting method. In the melt-extrusion method, a polymer is heated to be melted, and then extruded by an extruder, to form a film. The melt-extrusion method has advantages such as high productivity and relatively low equipment cost. However, in the melt-extrusion method, it is difficult to adjust the film thickness with high accuracy, and further fine streaks (die lines) easily occur on a surface of the film. Accordingly, it is difficult to produce a film having high quality as an optical functional film. On the contrary, in the solution casting film, a polymer solution containing a polymer and a solvent is cast onto a support to form a casting film. The casting film is hardened enough to be peeled and have a self-supporting property, and then peeled from the support to form a wet film. The wet film is dried to be a film. In the solution casting method, it is possible to obtain a film having more excellent optical isotropy and thickness evenness and containing less foreign substances in comparison with the melt-extrusion method. Therefore, the solution casting method is mainly adopted for a producing method of an optical functional film for use in the LCD or the like.
In the solution casting method, at first, a polymer such as triacetyl cellulose is dissolved into a mixed solvent containing dichloromethane or methyl acetate as a main solvent to prepare a polymer solution (hereinafter referred to as a dope). Then, a defined additive is mixed with the dope to prepare a casting dope. The casting dope is cast through a casting die onto a support such as a casting drum and an endless belt to form a casting film (hereinafter referred to as a casting process). The casting film is hardened enough to be peeled and have a self-supporting property on the support. Thereafter, the casting film is peeled as a wet film from the support. The wet film is dried and wound as a film.
Recently, in accordance with rapid increase in demand for the LCD and the like, a solution casting method having high production efficiency has been desired. In view of increasing the production efficiency, the speed at which the casting process is performed is slowest in the solution casting method. Therefore, for the purpose of speeding up the solution casting method, the moving speed of the support is made faster, and an upstream side from a casting bead in the moving direction of the support is decompressed by using a decompression means such as a decompression chamber. Note that, the casting bead is the casting dope extending from the casting die to the support.
During the casting process, when the clearance between the support and the decompression chamber is changed, the following problems occur in some cases. For example, in accordance with change in pressure inside the decompression chamber, a position of the support where the dope reaches is changed, and thereby thickness unevenness of the casting film occurs. Air enters between the casting film and the surface of the support in accordance with decrease in adhesion degree between the surface of the support and the casting bead. Accordingly, thickness unevenness of the film and defects on the surface of the film (surface undulation generated in the longitudinal and width directions of the film) occur. In view of the above, a film production apparatus as follows is disclosed in Japanese Patent Laid-Open Publication No. 2001-79864. In the film production apparatus, the clearance between the support and the decompression chamber is detected. When the clearance is less than a preset level, the decompression chamber is caused to move, to set the clearance between the support and the decompression chamber to the preset level or more.
Additionally, in a polymer film production method disclosed in Japanese Patent Laid-Open Publication No. 2002-103358, an air shielding plate or fin as an air shielding member is disposed at the vicinity of the casting die. In a cellulose ester film production apparatus disclosed in Japanese Patent Laid-Open Publication No. 2003-1655, the decompression chamber is provided with an adjustment plate as a labyrinth seal movable in a vertical direction, and in accordance with the vertical movement of the adjustment plate, the clearance between the adjustment plate and the surface of the support is adjusted.
Recently, in accordance with the variation in the usage of the film, it becomes necessary to produce films corresponding to each usage, namely the films whose material, thickness, and width are different from those of other films. According to the solution casting method for producing the films continuously as described above, when the film to be produced next has a different width, it is necessary to adjust not only a width of a discharge port for the dope in the casting die but also a width of an opening of the decompression chamber so as to correspond to the width of the casting bead. If the width of the opening is not adjusted in the solution casting method, flow of air at the vicinity of side ends of the casting bead causes vibration of the side ends of the casting bead. As a result, thickness unevenness and streaks occur on the film. Additionally, there is a limit for efficiently producing films having a width different from each other, because the casting device must be halted during the casting process for the purpose of adjusting the width of the opening.
According to the three patent documents described above, although it is possible to achieve a result at a certain level in view of preventing occurrence of thickness unevenness, still occurs thickness unevenness on the produced film. Accordingly, the three patent documents described above have a limit as a method for preventing occurrence of thickness unevenness.